Operating Systems

Interface between User & Computer Hardware

Applications Programs

Utilities

Operating System

Hardware

Utilities Memory Resident File Access & Control Program Creation Memory Resident File Format Structures o Editors Access Management o Compilers Protection Schemes o Debuggers System Access & Control File Manipulation System-Wide Access o File Manipulation Resource Access o File Deletion Error Detection & Response Mechanism Program Execution Error Detection Link-Loaders Error Correction Run-Time Management Response to Unrecoverable Error

I/O Device Access & Control Accounting Storage File Format Structures System Usage Collection Access Management System Performance Tuning Protection Schemes Forecasting Enhancement Requirements Billing Users for Usage

Resource Manager

O/S KernelKernel I/O Controller Printers,

Keyboards,

I/O Controller Monitors, Portions of Cameras, the O/S Etc. currently in

use

Computer Main System Memory Portions of Various I/O Application Devices

Programs Currently in use Operating

System Data

Application

Programs I/O Controller Storage

Processor Processor Data

Processor Processor

Operation Allocation of Main Memory is made jointly by both the O/S and Memory Management Hardware

O/S controls access to I/O devices by Application Programs

O/S controls access to and use of files

O/S controls access to and use of the processors, i.e., how much time can be allocated to the execution of a particular Application Program

Classification of Operating Systems

Interactive O/S Keyboard & Monitor Access to O/S Immediate, i.e., Interactive, Communication with the Program during Execution Phase

Batch O/S Multiple Programs from a diverse set of Users are batched together and submitted for execution Results are printed for each User Program to be read offline Example: Batch Updating of Bank Account Master Files at the conclusion of a Transaction Period (end of day)

Uniprogramming Single Program is loaded into Main Memory Processor concentrates on executing that program to completion

Multiprogramming Multiple Programs are loaded into Main Memory Processor executes the programs concurrently, i.e., switches between programs to maximize processor utilization

Muiltiprogramming

Uniprogramming Interactive Batch O/S O/S

Early Systems No Operating Systems Console o Set of Display Lights to indicate the state of the processor o Single Row of Display Lights used to indicate a single command written in binary numbers o Set of Toggle Switches to set the Display Lights for a single command written in binary code Input Device – Card Reader Program Creation o Written on Coding Pads o Transcribed to Hollerith Cards via a Card Punch Machine (cast iron) o Program in Hollerith Card Format read into the Main Memory by the use of a Card Reader o Programs were written to absolute memory locations o Entire program had to loaded into memory o Program could be written to Magnetic Tape by the use of a Tape Drive in write mode o Program could be retrieved from the magnetic tape by the use of a Tape Drive in read mode Program Execution o Initiated by a special “GO” switch on the console o Error condition – program halted; status could be investigated by inspecting registers and main memory o Normal Completion – output to printer or Card Punch

Scheduling Sign Up Sheet o Blocks of Time – 15 minutes o waste of processing time – sign up for more time than required o Inefficient use of time – need more time than allotted – forced to stop -- necessitates return at a different time – results in inefficient use of programmers time

Later Early Systems Setup Time o Single Program (job) o Loading Compiler o Loading High Level language Source Code o Saving the Object Code after compilation o Loading & Linking Program Object Code and Library Function Object Codes

Attaching/Detaching Loading/Unloading Magnetic Tape Drives Magnetic Tape Drives Card Readers Card Readers

Serial Processing – users had access to the computer System in Series

Software Tools o Libraries of Common Functions Interrupt Processing o Linkers Device Drivers o Loaders o Debuggers Memory Map Job Sequencing o I/O Driver Routines of a Resident Monitor Command Language Simple Batch Operating Systems (Monitors) Interpreter User o does not have direct access to the Computer System User Program Area o submits job to an operator (cards/tape) Operator o batches jobs together sequentially o submits entire batch to monitor for processing Monitor controls the sequence of events

Resident Monitor – portion of the Monitor that must always be in the main memory

Non-resident Monitor – portion of the Monitor that remains in storage until required o Utilities o Common Functions

Resident Monitor determines when to read in a new job

Monitor o allocates memory for a new job o reads a new job in from storage o inserts control information at end of job requiring job to restore control to the Monitor o passes control to the new job o at the end of the job, control passes back to the Resident Monitor

Monitor prints results of the previous execution for the User

Monitor schedules a new job

Processor (at a certain point in time) o is executing instructions from a portion of the Monitor residing in Main Memory that cause the next job to be read into Main Memory o encounters a branch instruction that leads the processor to start executing the new job o encounters either an end statement or an error condition  causing the processor to fetch its next instruction from specified locations in the Monitor code

Control is Passed to a Job Processor is fetching and executing instructions from the user code

Control is Returned to the Monitor Processor is fetching and executing instructions from the user code

Job Control Language (JCL) o provides control instructions to the monitor o FORTRAN . each instruction and each data item is coded on a separate  Hollerith card or  record on tape . $JOB -- indicates the beginning of a new job . $FTN -- specifies which FORTRAN compiler should be loaded  selected compiler produces object code – object code stored  in memory  “compile, load & go” operation  on tape  $LOAD card is required to place code in memory

$FTN passes control from Monitor to the FORTRAN compiler Upon termination of the compiler operation, control passes back to the Monitor

. if a $LOAD card is detected, the Monitor invokes the loader which loads the object code into the space previously held by the FORTRAN compiler; the Monitor transfers control to the object program . if a $LOAD card is not detected, the Monitor transfers control to the object program

Hardware Features (absent from early hardware) o Memory Protection . exclude user programs from writing in the Monitor memory space o Timer . prohibit any single job from executing beyond a specified time limit o Privileged Instructions . instructions provided for use by the Monitor code only . processor encountering a privilege instruction in a user program, passes control to the Monitor, which then executes an error interrupt o Interrupts . ability of the hardware to regain control from an executing program to process an exceptional situation

Processor time & memory space

are both shared by the  Utilization of the system is improved

user program & Monitor

Multiprogrammed Batch Systems

Idle Processor o CPU speed >> I/O device speed o single job available  CPU must wait for I/O completion

Multitasking o multiple tasks within the same program o processed concurrently

Multiprogramming o multiple processes from multiple programs o processed concurrently

Multiprogramming/Multitasking o multiple jobs are available for execution, CPU . encounters an I/O operation on current job . switches to alternate job while waiting for I/O completion

CPU Job A Job B Job C Job A Job B Job C Job A idle time

I/O Unit I/O A I/O B I/O C I/O A I/O B I/O C

Hardware Requirements o I/O Interrupt Processing Efficient Batch Processing o Direct Memory Access (DMA) o Memory Management

Time-Sharing Systems Direct User Interaction o Transaction Processing – reservation systems with the Executing Process o Multiple Interactive Jobs o O/S interleaves the execution of each user program in a short burst (quantum) of time

o N users  each user has effective control of a computer with slightly less than the speed of the actual computer (overhead costs of context switching)

Scheduling Long-Term Scheduling (infrequent exedcution) o determines which programs are admitted for processing . allocated memory space . allocated process control block (becomes a process) . process control block placed in the Ready Queue  batch system  PCB held on disk in a disk queue  some systems  PCB swapped-out to raw disk (no file management system) o considers the degree of multiprogramming & the size of remaining free memory

Medium-Term Scheduling o manages degree of multiprocessing o swaps jobs between raw disk memory Short-Term Scheduling (frequent execution) o decides which job to execute next o call the dispatcher, dispatcher executes the context switch between PCBs

Time-Sharing System – accepts all users until system is saturated; locks further entry Algorithm for Accepting New Processes – balance CPU bound versus I/O Bound Process States

New Ready Running Halt

Blocked

New – admitted by Long-Term Scheduler, PCB allocated, OS needs to allocate memory, etc Ready – process is waiting access to processor, i.e., PCB is complete Running – process is executing Waiting – process is suspended, waiting for some system resource, e.g., I/O completion Halted – process has terminated, awaiting deallocation

Process program code contents of the process control block

Process Control Block Identifier (unique integer) State Priority PC contents Memory Pointers – start & end locations of allocated memory space Context Data – contents of all other registers containing information important to the continued operation of the process I/O Status Information Accounting Information

Context Switch current user process issues a service call, e.g., I/O request o process is suspended until call is completed current user process issues an interrupt, i.e., hardware-generated signal o process is suspended until interrupt is processed o privileged instruction, divide by zero, time quantum interrupt event unrelated to current process o process is suspended until interrupt is processed

OS receives control of the at the interrupt handler if an interrupt occurred OS receives control of the at the service-call handler if an service call occurred

Process Scheduling Long Term Queue o disk resident o list of candidate jobs waiting to be selected by Long-Term Scheduler Short-Term Queue – Ready Queue Multiple I/O Queues – one for each I/O Device

I/O Scheduling – upon completion, OS moves process from queue to Ready Queue Selects a waiting process and signals I/O device to start execution Memory Management

Operating System

Process B

Process C

Process A

Swapping o Ready Queue may be empty – all the processes may be in the I/O queues o Swap processes from the I/O queues to . an Intermediate Queue on disk . swap space, i.e., raw disk – partition with no file system (faster swapping) o OS selects a process from . the Intermediate Queue to place in the Ready Queue . swap space, i.e., raw disk – partition with no file system (faster swapping)

Partitioning Holes in Memory o Fixed Size . Equal Size Areas of memory unusable because of small size . Unequal Size o Variable Size Compaction Moving processes to different memory locations to consolidate holes

Addresses o Logical Address . location relative to the beginning of the program (location zero) . program instructions contain logical addresses o Physical Address . actual location in main memory . base address  physical address of the current starting location of process in physical memory . processor adds the base address to the logical address to obtain the physical address

Paging pages – small fixed size chunks of program frames – small fixed size chunks of memory same size No external fragmentation of memory Internal fragmentation ½ of last frame Page tables maintain list of frames in use OS also keeps a list of free frames

Demand Paging Each page of a process is allocated a frame in memory only when it is referenced from a previous frame Page Fault Page Replacement Algorithms Trashing o repeatedly throwing out pages just before they were to be referenced Real Memory o actual physical memory Virtual Memory o perceived memory requirements of entire process o may be larger than size of real memory o exists on disk storage

Page Table Structure Memory-Resident Page Table Location (starting position) is held in a register Logical Address [page #, offset] Page # indexes into page table producing the frame # Physical Address [frame #, offset] Memory Management Unit – hardware required One page table / process

1 30 13 30 123 13

12 123 14

13 123 15

123 123 16

123 123 17

123 18 VAX Architecture 123 19 2 GBytes of Virtual Memory (231)  222 page table entries per process 512 byte pages (29) page tables are stored in virtual memory, i.e., on disk page table length page length

Two-Level Paging Scheme Page Directory o each entry points to a page table o if (page table length page length) && (directory length page length) then total pages (page length)2 Inverted Page Table index the page table by frame number rather than by page number use hash function to map the page number into a location in the page table hash collisions are maintained in a linked list, i.e., a chain of entries size of an inverted page table << size of normal page table

page # Offset Translation Lookaside Buffer miss hit real address

Tag Remainder Cache Page Table hit hit miss

Main Memory

Segmentation visible to programmer allows programmer to view memory multiple address segments segments are of dynamic size each segment may be assigned access and usage rights used to associate privilege & protection attributes to instructions & data memory reference (segment #, offset)

data structures may be assigned to a segment OS may expand or shrink the size of the segment as needed segments may be changed & recompiled independently segments may be shared among different processes

Pentium Memory Management Unsegmented Unpaged Memory – low-complexity high-performance controllers Unsegmented Paged Memory – BSD (Berkeley Unix Distributions) Segmented Unpaged Memory – predictable access times – embedded systems Segmented Paged Memory – Unix System V Distributions

Segmentation o virtual address (Pentium logical address) . segment reference  2-bit protection mechanism  14-bit segment specification . offset 32-bit o unsegmented memory  virtual memory = 232 bytes, i.e., 4 GBytes o segmented memory  virtual memory = 246 bytes, i.e., 64 TBytes . ½ virtual address space == global, i.e., used by all processes . ½ virtual address space == local, i.e., distinct for each process o physical address (32-bit address)  4 GBytes o segment protection . privilege levels  0 : kernel  memory management, protection, access control most protected  1 : kernel  remainder of OS modules  2 : specialized application subsystems with their own security mechanisms, e.g., DBMS, Office Automation, Software Engineering Environments  3 : application programs least protected . access attribute o Data Segment Privilege Level – Classification Program Segment Privilege Level – Clearance

Process may only access Data Segments for which its Clearance is lower than or equal to (more privileged or same privilege) as the Classification of the Data Segment